The present invention relates to a sensing device for sensing a biometric characteristic of a person""s hand, and comprising a matrix array of sensing elements defining a sensing area.
The use of biometric sensing and hand biometric sensing in particular as a method of verifying the identity of a person is well known. One example of a hand biometric sensing device is a fingerprint sensor.
Fingerprint sensing systems are attracting considerable interest for many different applications, ranging from high security uses such as access control for buildings and computers to low level security uses such as replacements for conventional locks and keys. Their main advantages for such purposes are that they are relatively easy and convenient to use, they avoid the need for keys, personal identification numbers and the like, and should be less susceptible to fraudulent use. The need for improving security and preventing fraudulent use, particularly, for example, when conducting financial transactions using banking terminals and ATMs and especially new applications arising from computer networking which include home banking and shopping on the internet, is widely recognised.
Examples of fingerprint sensing devices using matrix arrays of sensing elements are described in U.S. Pat. No. 5,325,442 (PHB 33628) and in the paper by N. D. Young et al entitled xe2x80x9cNovel Fingerprint Scanning Arrays Using Polysilicon TFTs on Glass and Polymer Substratesxe2x80x9d published in IEEE Electron Device Letters, Vol. 18, No. 1, January 1997. In these devices a capacitive sensing approach is utilised. The sensing elements are arranged in a two dimensional, row and column, planar array, occupying an area generally corresponding to the size of a fingertip, and addressed by sets of row and column address conductors. Each sensing element includes a sense electrode which together with an overlying part of a person""s finger and an intervening layer of insulator material, that provides a sensing surface on which the finger is placed, constitutes a capacitor. Charging of the sensing element capacitors, using signals applied to the address conductors, is controlled by switching devices in the form of thin film transistors (TFTs) in the sensing elements. The individual capacitances of these capacitors depend on the spacing of the sense electrodes from the overlying fingerprint portions so that ridges and valleys of a fingerprint produce different capacitances. By measuring the charge storage characteristics of the capacitors in the sensing elements an indication of the presence of a ridge or valley above each sensing element is obtained. The rows of sensing elements are addressed one at a time in turn so as to scan the fingerprint and the variation in sensed capacitances produced over the array by a fingerprint ridge pattern provides an electronic image or representation of the three dimensional form of the fingerprint surface.
These kinds of sensing devices are fabricated using the same kind of matrix switching thin film technology developed for active matrix display devices and the like to form the array of sensing elements and the sets of address conductors on a common substrate of glass or plastics and are considerably more compact and much less expensive to produce than conventional optical type fingerprint sensing devices which require a prism, lenses, a light source and a CCD chip or CMOS image sensor chip.
It is an object of the present invention to provide an improved sensing device of the kind using a sensing element array which is adapted to offer variable levels of security in use.
According to one aspect of the present invention, there is provided a sensing device as described in the opening paragraph which is characterised in that in a first portion of the array the sensing elements have a resolution capable of sensing the fingerprint pattern of a person""s finger when placed over that portion of the sensing area and in a second portion of the array the sensing elements have a lower resolution for sensing at least one other hand biometric characteristic. The ability of the sensing device, with its single sensing element array, to sense not just a person""s fingerprint but one or more additional biometric characteristics is important and advantageous. The performance of any hand biometric characteristic sensing system is based on system error rates, namely the false rejection rate and the false acceptance rate. For many applications employing a system which senses just one biometric characteristic the error rate is not satisfactory. By combining two, or more, biometric sensing systems the possibility exists for reducing the overall effective error rate but because most conventional biometric sensing systems tend to be expensive such combinations are not widely used and have been restricted to particular applications which justify the cost. The sensing device of the present invention, however, offers the ability to sense different hand biometric characteristics using just a single sensing element array for this purpose, thus simplifying manufacture and reducing cost. By using thin film technology to fabricate the sensing element array the device offers the advantage of compactness which is important when, for example, the sensing device is to be incorporated in other products, as well as being relatively inexpensive to produce.
The sensing elements may be of the kind described in the aforementioned patent specification and paper which use a capacitive sensing approach and comprise thin film switching devices such as TFTs. However, it is envisaged that sensing elements which are capable of responding to the presence of a hand surface part in proximity thereto in a different manner to provide an indication of hand biometric characteristics could be utilised. For example, the sensing elements may be responsive to pressure or skin resistance such that, in the first portion of the array, the sensing elements respond differently to the ridges and troughs of fingerprint pattern by virtue of the pressure, contact, or electrical resistance caused by the ridges when the finger is pressed over the array.
The other hand biometric characteristic which is sensed can be finger length and/or width, hand geometry, palm print or finger interxe2x80x94joint dimensions, all of which only demand lower (coarser) sensing resolution for their measurement compared with fingerprint pattern sensing. More than one of these may be sensed if desired. The data representative of the sensed characteristic or characteristics can be utilised together with the sensed fingerprint data to improve the accuracy of verification by comparing, with stored data, not just fingerprint information but this additional data as well. In practice, the biometric characteristics to be sensed could be selected depending on the level of security required, and the fingerprint sensing capability need not always be used. For lowxe2x80x94level security purposes, finger length/width or finger interxe2x80x94joint lengths may be sufficient whereas for high level security applications more than two biometric characteristics may be used, one preferably being fingerprint data.
By using a suitable stylus, adapted to interact with the sensing elements, the sensing elements could be utilised to detect dynamic movement of the stylus, thus enabling the device to be used also for scanning a person""s signature pattern for additional security.
The overall dimensions of the sensing element array will vary according to the particular characteristics to be sensed. An array adapted to sense only finger characteristics would, of course, be considerably smaller overall than an array for sensing hand geometry and could, for example, be conveniently incorporated in a smart card. In any event, the use of thin film technology enables differently sized arrays to be fabricated relatively easily. The provision of comparatively high and low resolution regions in the array leads to benefits both in manufacture and operation. In particular, fabrication is simplified, resulting in better manufacturing yields, the array can be driven at faster scanning rates, and the amount of drive circuitry needed to address the sensing elements will be less than that which would be required by a sensing element array having the same, high, resolution capability throughout.
In a preferred embodiment, the sensing elements of the array are arranged in rows and columns and connected to sets of row and column address conductors, each row of sensing elements being connected to a common row address conductor and addressed in turn by a drive circuit connected to the set of row address conductors, and the pitch of the sensing elements in the column direction, and the spacing between adjacent row address conductors, is greater in the second portion of the array than in the first portion. Such an array can easily and conveniently be fabricated at low cost using thin film technology to form the address conductors and sensing elements on a substrate of, for example, glass or plastics. In the first portion of the array the sensing elements in each column may be connected to a common column address conductor and in the second portion of the array the number of sensing elements in each row is less than in the first portion and the sensing elements are connected to certain ones only of the column address conductors associated with the sensing elements in the first portion, for example every nth column address conductor where n is a whole number greater than one. Where n, for example, equals 8, then the sensing element pitch in the row direction in the second portion will be one eighth of the sensing element pitch in the first portion.
The division of the sensing element array into high and low resolution portions leads to further advantages when using sensing elements of, for example, the kind described in the aforementioned publications. Because the number of sensing elements connected to each column conductor is small compared with that of an array having the same, high, resolution throughout, the parasitic column capacitance is lower which means that the noise performance of the read-out amplifiers connected to the column conductors is improved and signal attenuation is reduced. Also, less noise from the row drive circuit is coupled to the read-out amplifiers through the sensing elements due to the fact that fewer row conductors are needed.
Various other arrangements may be possible. For example, in one alternative arrangement the rows of sensing elements in the first portion may be physically shorter than the rows of sensing elements in the second portion with the column address conductors associated with the sensing elements in the first portion being fanned out at the transition between the two portions and each of these column address conductors being associated with a respective column of sensing elements in the second portion. In this case, the number of sensing elements in each row in both portions is the same and the pitch of the column conductors is constant in both portions but larger in the second portion. In the, first portion, the smaller sensing element pitch provides the resolution necessary for fingerprint sensing, the overall area occupied by the sensing elements in this portion being slightly larger than the overall size of a person""s finger tip, while the sensing elements in the second portion, being of larger pitch in both the row and column directions, occupy a considerably larger area, for example conforming with the size of a person""s hand. In this arrangement, however, the fanning out of the column address conductors will result in a dead region at the transition between the two portions.
According to another aspect of the present invention, there is provided a hand biometric characteristic recognition system comprising a sensing device in accordance with the one aspect of the present invention, processing means connected to the sensing device for receiving an output from the device indicative of the sensed hand biometric characteristics and comparing such with stored hand biometric characteristics data.